A holographic data storage and reconstruction system records an interference pattern in a storage medium, e.g., a photorefractive polymer, that is sensitively reactive to an amplitude of the interference pattern, and reconstructs the recorded interference pattern, wherein the interference pattern is produced by the interference of a reference beam and a signal beam that is generated from a target object. In such a holographic data storage and reconstruction system, the amplitude and phase of the signal beam are recorded therein by varying an incident angle of the reference beam, thereby reconstructing a three-dimensional shape of the object. Moreover, it is possible to store in the same storage medium a large quantity of page-based holographic data composed of binary data.
FIG. 1 shows a diagram of a conventional holographic storage and reconstruction system for storing in a holographic medium a three-dimensional hologram data (i.e., an interference pattern) produced by the interference of a signal beam with a reference beam and then reconstructing the three-dimensional hologram data stored in the holographic medium.
As illustrated in FIG. 1, the conventional holographic data storage and reconstruction system includes a light source 100 for emitting a laser beam and a holographic medium 114 for storing therein hologram data. Formed between the light source 100 and the storage medium 114 are two paths, i.e., a signal beam processing path S1 and a reference beam processing path S2, each including a plurality of optical systems.
Referring to FIG. 1, there is illustrated a beam splitter 102 for splitting an incident laser beam emitted from the light source 100 into a reference beam and a signal beam. The reference beam is reflected and then provided to the reference beam processing path S2 whereas the signal beam passes through the beam splitter 102 and then is provided to the signal beam processing path S1.
The reference beam provided to the reference beam processing path S2 is reflected by a mirror 108, and the reflected reference beam enters the holographic medium 114 via an optical system 112 composed of optical lenses.
Meantime, on the reference processing path S2, the mirror 108 operated by an actuator 110 deflects a reference beam at a predetermined angle, i.e., a recording angle or a reconstruction angle, and then provides the deflected reference beam to the storage medium 114. Herein, the reconstruction angle used for reconstructing the recorded data should be identical to the recording angle used for recording the data.
Provided on the signal beam processing path S1 are sequentially a spatial light modulator (SLM) 104, a mirror 106 and an optical lens 107 in a direction of the signal beam incident on the holographic medium 114.
On the signal beam processing path S1, the SLM 104 modulates the signal beam into a page-based data composed of a plurality of pixels representing binary data based on an externally inputted data. The modulated signal beam is then reflected by a mirror 106 and then provided to the holographic medium 114 via the optical lens 107.
In this way, the signal beam and the reference beam entering the holographic medium 114 respectively along the signal beam processing path S1 and the reference beam processing path S2 interfere with each other in the holographic medium 114. Depending on the amplitude of the interference pattern produced by the interference, a photo-induced transformation occurs in the holographic medium 114, thereby the interference pattern being recorded in the holographic medium 114.
The data recorded in the holographic medium 114 during the above-described holographic data recording process is reconstructed as follows.
That is, if the reference beam, i.e., the reconstructing reference beam, is illuminated on the holographic medium 114 in order to read out the data recorded in the holographic medium 114, the reconstructing reference beam is diffracted by the interference pattern recorded in the storage medium 114. As a result, the page-based data (signal beam) composed of a plurality of pixels representing binary data is restored, and an image of the restored signal beam is captured by a CCD 116.
The CCD 116 converts each pixel level of the captured image (page) into a series of m-bit binary data and then outputs the binary data to a splitter 118. The splitter 118 removes a least significant a (0≦a<m) bits from the m-bit data outputted from the CCD 116 and then outputs a remaining m−a bits. Herein, although the data outputted by the CCD 116 is assumed to be digital data, the output data can be an analog data. In case the output data is the analog data, an A/D converter can be additionally connected between an output of the CCD 116 and the splitter 118.
The m−a bit data outputted by the splitter 118 is converted into binary data ‘0’ or ‘1’, thereby ultimately restoring the page-based data composed of the binary data. For example, such conversion can be carried out by applying a modulation code to the m−a bit data. Further, by comparing the m−a bit data with a predetermined threshold, in case the m−a bit data is greater than the threshold, the data is converted into a binary data ‘1’ whereas in case the m−a bit data is smaller than or equal to the threshold, the data is converted into a binary data ‘0’.
The data reconstructed by the conventional apparatus for storing and reconstructing the holographic data is an image data composed of 1K to 1M pixels included in hundreds to thousands of frames per second. However, the holographic data reconstruction system should restore the original page-based data by processing the outputted image data within a limited time.
The amount of data to be processed by the holographic data reconstruction system increases in proportion to the number (m) of bits of data outputted by the CCD 116. Therefore, it is preferable to reduce the amount of data to be processed by increasing the number of the least significant a bits removed from the m-bit data outputted by the CCD 116. However, the conventional holographic data reconstruction system has a drawback in that the bit error rate (BER) of binary data representing the outputted page-based data increases as the number (a) of the removed bits increases.